This is why Cranelift is being worked on. There is also a Rust interpreter, miri.

I think starting with LLVM was the right decision (and one that was I was primarily responsible for). Rust would lose most of its benefits if it didn't produce code with performance on par with C++. LLVM is not the fastest compiler in the world (though it's not like it's horribly slow either), but its optimization pipeline is unmatched. I don't see replicating LLVM's code quality as feasible without a large team and a decade of work. Middling code gen performance is an acceptable price to pay until we get Cranelift; the alternative, developing our own backend, would mean not being able to deploy Rust code at all in many scenarios.